The present invention relates to the field of overvoltage protection circuits for lines such as telephone lines.
Among the known overvoltage protection circuits, circuits of the type shown in FIG. 1A are now frequently adopted, wherein protection is ensured by two bidirectional components P1, P2 connected between each conductor and ground, each component having the electrical characteristics shown in FIG. 1B. When overvoltage occurs on one of the conductors, the protection components goes from a blocked state (high impedance) to a conductive state (low impedance) thus shorting the conductor with ground. However, in standby state (high impedance) each of the two components withstands the conductor/ground voltage. Usual voltage values are -48 V for one conductor and about 0 V for the other. This voltage asymmetry generates (as explained later on) a capacitance asymmetry of the components. This asymmetry, permissible for usual networks, is a major impediment for some digital network lines.
Other protection circuits have been proposed, amongst others the one of FIG. 2, wherein three unidirectional protection units 1, 2 and 3 are connected between a common point C and a first conductor A, a second conductor B and ground G. Each protection component comprises in anti-parallel arrangement an unidirectional protection component T and a diode D, respectively T.sub.1, D.sub.1 ; T.sub.2, D.sub.2 ; T.sub.3, D.sub.3. In FIG. 2, the cathodes of diodes D.sub.1, D.sub.2, D.sub.3 are interconnected and the anodes of the protection components T.sub.1, T.sub.2, T.sub.3 are interconnected.
Such a structure is for example described in U.S. Pat. No. 4,282,555.
The protection components T.sub.1, T.sub.2, T.sub.3 are, for example, gateless thyristors which become conductive from their anode to their cathode when the voltage across their terminals exceeds a threshold value predetermined during manufacturing. However, it is possible to choose other known protection components, for example, components of the avalanche diode type or thyristors, the gate of which receives a bias signal, which is insufficient to render them conductive but sufficient to adjust their breakdown threshold between main terminals.
An advantage of this structure, when the protection components T.sub.1 are gateless thyristors, is that it is easily integrable on two, or even one, chips.
The invention relates more particularly to the capacitive behavior of circuits of the type shown in FIGS. 1 and 2.
The thyristor-type components, or more generally other types of unidirectional protection components, generally have a substantial capacitance between their main electrodes. This capacitance is well above that of conventional diodes such as diodes D.sub.1, D.sub.2 and D.sub.3. Since the diodes are arranged in parallel with the protection components, the influence of their own capacitance can be neglected.
Thus, the provision of protection components changes the capacitance between each conductor A or B and ground G. This is unimportant when the lines are intended to carry low frequency electrical current or telephone signals, but becomes a major drawback when the lines carry digital signals corresponding to binary transitions between two states, the transitions having to be as steep as possible. In that case, the presence of relatively high parasitic capacitances, and especially a capacitance dissymmetry between the conductors, is a major drawback. These capacitances cause distortion of signals and may even render them indiscernible.
Indeed, although the circuit of FIG. 2 is symmetrical as regards the suppression of overvoltages on either conductor A or B, it is asymmetrical as regards its capacitive behavior. This asymmetry has been more particularly noted in the case indicated as preferred in the above patent, namely, the protection of telephone lines.
In the prior art, to avoid this problem, it has been necessary to use symmetrical protection structures both for the protection function and for the capacitive behavior. An example of such a structure is shown in FIG. 3. In this structure, three diodes D11, D12, D13 are connected by their cathodes to a first terminal of a bidirectional overvoltage-triggered protection component T10, the three diodes being connected by their cathodes to conductor A, conductor B and ground G, respectively. Three additional diodes D14, D15, D16 are connected by their anodes to the second terminal of component T10 and by their cathodes to conductor A, conductor B and ground, respectively. With this structure, whatever the polarization of conductors A and B with respect to ground, component T10 has the same capacitance and, moreover, this capacitance is low because T10 is in a biased state. The drawback of this structure is the difficulty encountered, on the one hand, for manufacturing a bidirectional overvoltage-triggered component that is really symmetrical and, on the other, for integrating the overall structure as a monolithic component.
Despite this drawback, the type of structure shown in FIG. 3 is used when conductors A and B are intended to transmit digital signals.